This invention relates to conductive materials and is particularly concerned with conductive materials to serve the purpose of a heating means.
It has long been known that materials that are electrically conductive can generate heat. This phenomenon has spawned a considerable number of heating means for a considerable number of different purposes.
Whilst electrical heaters at large are generally successful, and more than capable of meeting their intended purpose, what has proved to be difficult, is the achieving of a uniform heating effect over a relatively wide area, substantially free from hot and cold spots or areas.
There have been attempts hitherto to create electrical heaters in sheet or web form to serve a wide variety of purposes. However, the most usable form of conductive material is in the form of carbon particles or carbon black embedded in or coated on a carrier or substrate. To attempt to achieve a uniform heating effect, a greater density or concentration of carbon particles is employed, but as a direct result of that, the material becomes less pliable and more brittle, to the detriment of the employment of the material over a wide range of potential uses.
The first object of the present invention is to provide a start or batch material able to be used in a variety of different physical forms to suit a wide variety of electrical conduction heaters.
Second and subsequent objects of the invention are to transform the start or batch material into products that are electrical conductive heaters for a wide variety of different purposes.
According to a first aspect of the present invention, a conductive material comprises finely divided carbon particles uniformly dispersed in an elastomeric carrier there being carbon particle levels of 20% by weight to 75% by dry weight to 80% to 25% by dry weight of carrier levels.
Carbon particle levels of 20% to 40% by dry weight to 80% to 60% by dry weight of carrier levels can be used.
The carbon particles may be so-called carbon black, a finely divided powder, and the elastomeric carrier is an elastomeric polymer, and according to a further feature of the invention, a semi-conductive film or coating comprises a carbon filled elastomeric polymer, preferably with carbon particle levels of 43% to 73% by dry weight to 57% to 27% by dry weight polymer levels, preferably 60% by dry weight to 65% by dry weight. Further preferably, the carbon particle level is 57% by dry weight.
The elastomeric polymer may be an aliphatic polyurethane in solution, and desirably the carbon particles are mixed without milling prior to the addition thereto of the polyurethane solution.
If required in particular applications, flame retardant materials may be provided.
According to a further aspect of the invention a method of forming a compound for an electrically conductive heater comprises stirring fine carbon particles into a polymer base containing an anti-adsorption compound, to achieve carbon particles to polymer levels of 20% by dry weight to 75% by dry weight to 80% to 25% by dry weight of polymer, and subjecting mixture to high speed stirring for a pre-determined period of time, with the maintenance of the mixture below a predetermined level, to grind the carbon particles to a predetermined final fineness. The predetermined level may be at not more than 25xc2x0 C.
The required carbon particle level may be 20% to 40% by dry weight to polymer level but preferably carbon levels are from 43% to 73% by dry weight to polymer, and more particularly 57% by dry weight.
The resultant mixture exhibits thixotropic rheological properties, and if required, the viscosity of the mixture can be reduced, by the addition of a suitable solvent such as dimethylformamide.
Preferably the carbon black particles have particulate size of approximately 30 En nm. Suitable adsorbants may be selected from the group containing polypropylene glycols, and polyethylene glycols of a required molecular weight. After this process is complete a suitable polymer solution eg an aliphatic polyurethane is added to the master batch such that the ratio of polymer to carbon black is 1:0.57 on a dry basis. The finished compound is then refiltered prior to any coating process.
The duration of the slow stirring-in of carbon black is not critical, but desirably high speed stirring is limited to not more than 30 minutes.
Once the mixture is prepared, it is checked by taking a sample and passing it through a 200 mesh filter, preferably with the aid of a low pressure displacement pump, and checked by using a Heckman gauge, to ensure that there has been no agglomeration of the carbon particles during mixing. If any agglomeration is detected, the mixture should be subjected to further high speed stirring.
Preferably, the prepared start or batch material is subjected to a final filtration step by passing the mixture through a 300 mesh filter cloth, for example by way of a slow, low pressure positive displacement pump, at which stage there should be no residue left on the filter cloth that would signal that there was still a degree of remnant agglomeration of carbon particles.
In the form where the start or batch material employs dimethylformamide as the solvent and polyethylene or propylene glycol as the polymer base, it constitutes an ideal material to serve as a coating or a base or carrier material.
According to a further aspect of the invention a web or sheet is formed by applying a quantity of finished compound as discussed above to a release paper by way of transfer coating, to achieve a uniform coating or film of compound between 90 and 100 grams per square meter dry weight, and subjecting the web or sheer to heat progressively rising from 110xc2x0 C. to 150xc2x0 C. to achieve the controlled release of solvents and provide a coating or film free of pinholes.
As the electrical conductivity, and hence the heating effect achievable is a function of coating or film thickness the above process is repeated until a desired thickness of coating or film is created.
At this stage considerable care must be exercised to ensure that reticulation is avoided, and as it is preferred to spread the finished compound on the release paper by employing a doctor blade, equally considerable care exercised to ensure the avoidance of the presence of dirt or grit on the blade edge, to prevent the creation and spread of lines of indentations in the coating or film.
Desirably, the release paper is matt grade and is an unembossed silicone-coated paper.
According to a still further aspect of the invention, a flexible fabric able to serve the purpose of an electrical conductive heater is formed by taking the release paper mentioned above with its coating of finished compound spreading thereon a further quantity of said compound, laying the release paper on a flexible fabric carrier sheet or web, and passing the composite through a fixed gap roller to ensure controlled penetration of said compound into the fabric of the sheet or web, the sheet or web thereafter being subjected to heat progressively rising from 110xc2x0 C. to 150xc2x0 C. to achieve controlled release of solvents and provide a coating of film free of pinholes. Also possible is the direct application of a coating of finished compound directly on to a fabric carrier.
Such a sheet or web can be of any desired length, and of any width with the limits of available fixed gap rolls.
By the nature of the coat or film, there is a substantially totally even spread of carbon black at loadings in the polymer material considerably beyond that which has hitherto been believed to be possible whilst at the same time leaving the fabric with its film or coat totally flexible.
The fabric may be a knitted cotton material but can be of any other suitable form, such as a weft knitted polyvinyl alcohol fabric.
The preparation of the said compound and the manner of its application to a carrier is such that a visually smooth coating or film is provided. However, it remains so that the coating will exhibit a microscopic degree of roughness with peaks and troughs formed by carbon particles at the exposed surface. It is then important to pay special attention to the provision of an electrical connection to the coat or film.
According to yet another aspect of the invention an electrical connection to a coat or film incorporating carbon particles is formed by first spraying a nickel compound to an area of the coat or film, and applying to the sprayed area a tin-copper tape coated with a silver loaded conductive adhesive. With the electrical connection installed, the sheet or web can be connected to a suitable power source with the substantial guarantee that there will be no shorting or arcing at the point of electrical connection, and consequently no damage by the creating of hot spots.
Dependant on the use to which the sheet or web is put, an electrical connection can be provided over long lengths to opposite edges, to assist in the even input of power to the coat or film over a wide area, by locating a conductive rail on the silver loaded conductive tape. Desirably, the conductive rail is overlaid by an antifaying compound, preferably wider than the rail.
An important aspect of the invention is that the totally uniform heating effect achievable in the coat or film can be with relatively low power, eg 24 volts, and by controlling the width between the electrodes or rails, and the thickness of the coat or film, a constant temperature can be achieved and maintained, at any required level to serve a particular purpose.
In its form as a flexible sheet or web, an outer insulating layer can be sprayed on to form a water/fluid resistant electrical insulator. Suitable materials may be polyurethane, silicone or acrylic elastomers. The invention has a considerable number of available uses. It can be wrapped round an article the temperature of which is to be maintained and equally can be incorporated into clothing for use in extremely cold climates, to maintain the temperature of the wearer.
However, the fabric has significant benefits when used in medical contexts. It can be incorporated in a mattress or as a blanket for an operating table or for a bed in the recovery ward for raising and maintaining the temperature of patients following surgery.
It is possible to employ the fabric in sleeping or carrying bags for use by rescue services to give immediate aid to accident victims suffering from hyperthermia.
In all such uses, the low voltage required means that there is total safety to the user. The voltage and coat or film thickness can determine the maximum and constant temperature across the full width and length of the fabric.
The arrangement may be that power is supplied to the electrically conductive heater until such time as its required temperature is reached and then maintained at that temperature by an appropriate switching arrangement to switch power ON and OFF as required. It is however preferred for power to be supplied as a series of pulses of predetermined time, with intervening periods where power is switched off for predetermined periods of time, to allow temperature sensing to take place. Irrespective of the form of power supply, the invention allows highly efficient use of electrical power.
In addition to being able to control maximum temperatures by voltage control by control of the spacing between electrodes and by coating thickness it is a highly advantageous feature of the invention that the predetermined maximum temperature to suit the application is achieved in reasonable time from the onset of power, after which the temperature is regulated and maintained at its predetermined level. This can be of considerable importance not only in medical contexts but also in such as the food industry and particularly in food processing where a required temperature must be achieved quickly and maintained.
In addition to its use in conjunction with a flexible fabric, the said compound can be used differently. For example, it may be sprayed onto products whereby to provide heat uniformly over the whole surface of a complex product. The said compound can be screen printed onto support surfaces, or can be directly coated on to a product surface. Equally it can be calendered or hot melt coated from dry compound to produce flexible sheets, or can be applied by a powder coating technique to produce heatable laminates.
A further possibility within the invention is to provide a said compound where the polymer material into which the carbon black is stirred is such as to make the compound suitable for moulding or casting. Thus, form-stable sheets or shapes of material can be produced that can be used in a number of industrial applications, such as sub floor heater pads, sub soil heater pads, linings for fly ash hoppers in power stations the temperature of which needs to remain constant to avoid the ash from becoming damp, or placement around such as pump and valve castings, to prevent them from freezing and hence malfunctioning. In addition the said compound can be moulded around vessels for warming chemical or liquids.